Organic light-emitting device and method of fabricating the same

ABSTRACT

An organic light-emitting device, comprising: a substrate; a first conductive layer formed over the substrate; at least one layer of a light-emissive organic material formed over the first conductive layer; a barrier layer formed over the at least one organic layer which acts to protect the at least one layer of organic material; and a second conductive layer, preferably a patterned sputtered layer, formed over the barrier layer.

This is a continuation of application Ser. No. 09/180,037, filed May 6,1999 and issued as U.S. Pat. No. 6,541,790, which claims priority from(1) PCT/GB97/01208 filed May 2, 1997, and (2) GB 9609282.0 filed May 3,1996, each of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to organic light-emitting devices, inparticular patterned or pixelated organic light-emitting diodes, and amethod of fabricating the same.

Organic light-emitting devices (OLED's) such as described in our earlierU.S. Pat. No. 5,247,190 or in Van Slyke et al.'s U.S. Pat. No. 4,539,507have great potential for use as monochrome and multi-colour displays.OLED's based on semiconductive conjugated polymers are described in ourearlier U.S. Pat. No. 5,247,190, the contents of which are incorporatedherein by reference. Principally, an OLED consists of an anode whichinjects positive charge carriers, a cathode which injects negativecharge carriers and at least one organic electroluminescent layersandwiched between the two electrodes. Typically, the thickness of theat least one organic layer is of the order of 100 nm and the electricalconductivity of the material of the at least one organic layer issufficiently low as to avoid current spread from the overlap areabetween the cathode and the anode. Thus, light emission from the atleast one organic layer occurs only where the cathode and the anodeoverlap and therefore pixelation and patterning is achieved simply bypatterning the electrodes. High resolution is readily achieved and isprincipally limited only by the overlap area of the cathode and theanode and thus by the size of the cathode and the anode. Dot-matrixdisplays are commonly fabricated by arranging the cathode and the anodeas perpendicular arrays of rows and columns, with the at least oneorganic layer being disposed therebetween.

Low resolution dot-matrix displays can, for example, be fabricated bycoating at least one organic electroluminescent layer onto a substratehaving thereon an array of indium-tin oxide (ITO) lines which act as ananode. A cathode comprising an array of lines perpendicular to those ofthe anode is provided on the other side of the at least one organiclayer. These cathode lines may, for example, be lines of aluminium or analuminium-based alloy which can be evaporated or sputtered through aphysical shadow mask. However, shadow masking may not be desirable forvarious reasons. In particular, there are significant constraints on theuse of shadow masks when displays of large area and/or high resolutionare required. In order to produce such electrode line arrays and otherpatterns of large area and/or high resolution one would normally have touse various forms of lithography.

In order to fabricate efficient and stable OLED's with the desiredelectrical and light output characteristics great care must normally betaken in the design and construction of the interfaces between anyorganic layer and the electrodes. The particular importance of theseinterfaces is due to the fact that charge carriers should be injectedefficiently from the electrodes into the at least one organic layer.

Maintaining the desired electrical and light output characteristics ofthe pixels in an OLED display when lithographic processes are used tofabricate the electrode patterns, in particular where those patterns areon top of the at least one organic layer, is not trivial owing to therisk of the lithographic processes modifying and potentially damagingthe organic layer/electrode interfaces and the vicinity. Such damageduring lithography may originate from the photoresists, the developers,the etching processes (both dry and wet, negative and positivetechniques and etch and lift-off) or the solvents used. It should bementioned here that conjugated polymers are often deposited from and aresoluble in organic or aqueous solvents.

Plasma etching/ashing is very often used in lithography to remove thephotoresist or residual photoresist which may not have been washed offby the developer. Organic electroluminescent and charge transportingmaterials would normally be damaged, modified and/or etched very rapidlyin such dry etching/ashing processes if directly exposed to the plasma.

It is an aim of the present invention to provide an efficient organicelectroluminescent device that has a construction which allows for theuse of various lithographic processes to form the electrode on top of atleast one organic layer without significantly changing the electricaland light output characteristics of the display.

SUMMARY OF THE INVENTION

The present invention provides an organic light-emitting device,comprising: a substrate; a first conductive layer formed over thesubstrate; at least one layer of a light-emissive organic materialformed over the first conductive layer; a barrier layer formed over theat least one organic layer which acts to protect the at least one layerof organic material; and a patterned second conductive layer formed overthe barrier layer. In one embodiment the second conductive layer issputter deposited. In another embodiment the second conductive layer isevaporated.

The present invention also provides an organic light-emitting device,comprising: a substrate; a first conductive layer formed over thesubstrate; at least one layer of a light-emissive organic materialformed over the first conductive layer; a barrier layer formed over theat least one organic layer which acts to protect the at least one layerof organic material; and a sputtered second conductive layer formed overthe barrier layer.

In one embodiment the first conductive layer is the anode and the secondconductive layer is the cathode.

In another embodiment the first conductive layer is the cathode and thesecond conductive layer is the anode.

At least one of the two electrodes is at least semi-transparent.Preferably, the anode is light-transmissive. More preferably, the anodecomprises indium-tin oxide, tin oxide or zinc oxide.

Preferably, the anode has a thickness in the range of from 50 to 200 nm.

Preferably, the cathode comprises Al or an alloy thereof.

Preferably, the first conductive layer is patterned.

Preferably, the at least one organic layer is patterned.

Preferably, the organic material is a conjugated polymer.

Preferably, the thickness of the at least one organic layer is about 100nm.

Preferably, the barrier layer has a thickness in the range of from 1 to10 nm. More preferably, the barrier layer has a thickness in the rangeof from 2 to 5 nm.

Preferably, the sheet resistance of the barrier layer is at least 1MΩ/square.

Preferably, the barrier layer is a continuous layer.

Preferably, the barrier layer comprises a dielectric.

In one embodiment the dielectric comprises an inorganic oxide,preferably one of an oxide of Al, Ba, Ca, Mg, Ni, Si, Ti or Zr, an oxideof an Al alloy, or an oxide of Al—Li or Al—Mg. The oxide compositiondoes not have to be stoichiometric. Preferably, the oxide issub-stoichiometric.

In another embodiment the dielectric comprises a carbide, preferably acarbide of Hf, Mo, Nb, Ta, Ti, W or Zr.

In a further embodiment the dielectric comprises a boride, preferably aboride of Cr, Mo, Nb, Ti, W or Zr.

In a yet further embodiment the dielectric comprises a nitride,preferably a nitride of Ti or Zr.

In a yet still further embodiment the dielectric comprises a fluoride,preferably a fluoride of Ca or Mg.

Preferably, the substrate comprises a glass or a plastics material.

The present invention further provides a method of fabricating anorganic light-emitting device, comprising the steps of: forming a firstconductive layer over a substrate; forming at least one layer of alight-emissive organic material over the first conductive layer; forminga barrier layer over the at least one organic layer; and forming apatterned second conductive layer over the barrier layer; wherein thebarrier layer acts to protect the at least one organic layer duringsubsequent processing steps. In one embodiment the second conductivelayer is deposited by sputtering, preferably by DC magnetron or RFsputtering. In another embodiment the second conductive layer isdeposited by evaporation, preferably by resistive or electron-beamthermal evaporation.

In one embodiment the step of forming the patterned second conductivelayer comprises deposition through a shadow mask.

In another embodiment the step of forming the patterned secondconductive layer comprises the steps of: forming a layer of aphotoresist over the barrier layer; patterning the layer of photoresistto expose regions of the barrier layer where the second conductive layeris to be formed; depositing a conductive layer over the patterned layerof photoresist; and removing the regions of the conductive layer whichoverlie the patterned layer of photoresist.

Preferably, the method further comprises, prior to the step ofdepositing the conductive layer, a plasma cleaning step to remove anyresidual photoresist.

In a further embodiment the step of forming the patterned secondconductive layer comprises the steps of: forming a layer of conductivematerial; forming a layer of a photoresist over the layer of conductivematerial; patterning the layer of photoresist to expose regions of theconductive layer; removing the exposed regions of the conductive layer;and removing the photoresist.

The present invention still further provides a method of fabricating anorganic light-emitting device, comprising the steps of: forming a firstconductive layer over a substrate; forming at least one layer of alight-emissive organic material over the first conductive layer; forminga barrier layer over the at least one organic layer; and forming asputtered second conductive layer over the barrier layer; wherein thebarrier layer acts to protect the at least one organic layer duringsubsequent processing steps. In an embodiment the second conductivelayer is deposited by DC magnetron or RF sputtering.

In one embodiment the step of forming the patterned second conductivelayer comprises deposition through a shadow mask.

In another embodiment the step of forming the patterned secondconductive layer comprises the steps of: forming a layer of aphotoresist over the barrier layer; patterning the layer of photoresistto expose regions of the barrier layer where the second conductive layeris to be formed; depositing a conductive layer over the patterned layerof photoresist; and removing the regions of the conductive layer whichoverlie the patterned layer of photoresist.

Preferably, the method further comprises, prior to the step ofdepositing the conductive layer, a plasma cleaning step to remove anyresidual photoresist.

In a further embodiment the step of forming the patterned secondconductive layer comprises the steps of: forming a layer of conductivematerial; forming a layer of a photoresist over the layer of conductivematerial; patterning the layer of photoresist to expose regions of theconductive layer; removing the exposed regions of the conductive layer;and removing the photoresist.

The present invention also extends to the use of a barrier layer in anorganic light-emitting device which includes at least one layer of anorganic material arranged between first and second conductive layerswhich act as the electrodes for the device, wherein the barrier layeracts to protect the at least one organic layer against subsequentprocessing steps.

For the purpose of the invention it is important that the barrier layerhas sufficient electrical resistance to prevent current spread in thedevice, particularly between pixels of the device. A thin layer of sucha barrier layer, for example dielectrics such as metal oxides, arrangedbetween the cathode and the adjacent organic layer has been found toimprove the efficiency of OLED's as shown for example in JP-A-5-3080.

The provision of a continuous barrier layer establishes a well definedinterface between the upper electrode and the organic layer. Theinterface between the upper electrode and the organic layer can becontrolled by various processes, for example dry or wet cleaning, towhich the barrier layer is much more resistant than the organic layer.In the case where inorganic materials such as the above-mentioned metaloxides are used as the barrier layer, the barrier layer acts as a veryefficient etch-stop. Edge effects at the edges of pixels, which couldoccur when a patterned electrode is directly in contact with theadjacent organic layer, are also reduced if not eliminated with theintroduction of a continuous barrier layer.

Such methods of fabricating organic light-emitting devices providedevices of improved performance and stability when the upper electrodedeposited over the organic layer is a patterned layer. Preferably, theorganic layer is deposited by way of evaporation (resistive orelectron-beam) or sputtering (reactive or non-reactive). The preferredmethod, for example in the case where an inorganic metal oxide is used,is DC magnetron sputtering from a metal or alloy target in the presenceof oxygen such that the stoichiometry can be readily controlled and thedesired electrical properties can be achieved. RF sputtering from adielectric target is also a possibility.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be describedhereinbelow by way of example only with reference to the accompanyingdrawings, in which:

FIG. 1 illustrates an organic light-emitting device in accordance with apreferred embodiment of the present invention;

FIG. 2 illustrates the optical absorption characteristics ofpoly(p-phenylene vinylene) without a thin aluminium-lithium oxidecoating layer as function of etch-time in an argon-oxygen plasma; and

FIG. 3 illustrates the optical absorption characteristics ofpoly(p-phenylene vinylene) with a thin aluminium-lithium oxide coatinglayer as function of etch-time in an argon-oxygen plasma.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The organic light-emitting device comprises a substrate 1, an anode 3formed over the substrate 1, a layer 5 of a light-emissive organicmaterial formed over the anode 3, a barrier layer 7 formed over theorganic layer 5, and a cathode 9 formed over the barrier layer 7.

The substrate 1 is a piece of glass, preferably having a thickness ofless than 1.1 mm. The anode 3 is a patterned array of lines, preferablyof indium-tin oxide having a thickness of from 50 to about 200 nm. Theorganic layer 5 is a layer of poly(p-phenylene vinylene) (PPV), anorganic conjugated polymer as described in our earlier U.S. Pat. No.5,247,190. The organic layer 5 preferably has a thickness of the orderof 100 nm. The barrier layer 7 is a continuous layer ofaluminium-lithium oxide of about 3.5 nm in thickness. The cathode 9 is apatterned array of lines.

The organic light-emitting device is fabricated in the following way.The substrate is coated with a thin layer of a conductive material,preferably indium-tin oxide of from about 50 to about 200 nm. Theconductive material is patterned as an array of lines by way of standardwet-chemical etching. The etched structure is then, after cleaning,overcoated with a layer of poly(p-phenylene vinylene) having a thicknessof about 100 nm. This structure is then overcoated with a continuouslayer of aluminium-lithium oxide by DC magnetron sputtering from analuminium-lithium target with a lithium content of from about 3 to about5% in the aluminium to a thickness of typically 3.5 nm. Oxygen is mixedinto the argon sputter gas in a ratio of about 4 (argon) to 1 (oxygen)in order to oxidise the aluminium-lithium while it is deposited on thesubstrate 1. It has been found that the exact stoichiometry of theoxide, although important for achieving efficient devices, is notcrucial for the purpose of the present invention, namely to protect theorganic layer from subsequent processing steps and act as an etch stop,as long as the sheet resistance of the barrier layer 7 is low enough toprevent intolerable current spread between neighbouring lines of thecathode 9. In this context intolerable here means that neighbouringpixels which are in the off state are not accidentally switched on dueto the spread of current.

The patterned cathode 9 can be formed on the barrier layer 7 by variousmeans, three examples of which are given below.

Firstly, the patterned cathode 9 can be formed by evaporation orsputtering through a shadow mask consisting of closely spaced fine lineswhich is placed in close proximity to the structure during deposition.

Secondly, standard lift-off photolithography can be employed to form thepatterned cathode 9. Here, photoresist is coated the over the barrierlayer 7 and then patterned/exposed and developed such that thephotoresist is washed away where the cathode 9 is to be formed. A shortargon-oxygen plasma clean can be used to clean off residual photoresistfrom the barrier layer 7. It has been found that neither this wetprocess nor the plasma clean damage the thin barrier layer 7 or theunderlying organic layer 5. The material of the cathode 9 is thenevaporated or sputtered over the etched area and the photoresist withthe overcoated material is washed off (lifted-off).

The etch-stop property of a thin aluminium-lithium oxide layer is shownin FIG. 3. The traces in FIG. 2 show the optical absorptioncharacteristics of an uncoated film of poly(p-phenylene vinylene) on aglass substrate before (t=0 sec) and after various periods of exposureto an argon-oxygen dry plasma etch. The poly(p-phenylene vinylene) isetched away significantly, in particular after prolonged exposure to theplasma. The traces in FIG. 3 show an equivalent film of poly(p-phenylenevinylene) on a glass substrate but with an overlying layer of about 3.5nm of aluminium-lithium oxide deposited thereon by reactive DC magnetronsputtering from an aluminium-lithium target. The traces show veryclearly that the thin oxide layer is very efficient in protecting thepoly(p-phenylene vinylene) layer from the plasma etch, i.e., the oxidelayer acts as an etch-stop.

Thirdly, standard etch photolithography can be employed to form thepatterned cathode 9. Here, the barrier layer 7 is coated with acontinuous layer of the material for the cathode 9. Then, a photoresistis spun on top, patterned/exposed and developed. On the areas wherethere should be no cathode material the layer is etched away (dry orwet-chemical etching) and finally, after etching is complete, theresidual photoresist is removed.

In all these cases, the continuous thin barrier layer acts as aprotective layer and etch stop during the patterning process. It alsoacts as a buffer layer to reduce or eliminate possible edge effects atthe edges of the pixels since these edges are not in direct contact withthe organic layer. Without the barrier layer there would be a muchgreater risk of damaging the at least one organic layer and theinterface with the top electrode where the top electrode is, forexample, patterned to fabricate dot-matrix displays. Furthermore, thethin barrier layer also protects the organic layers to some degreeagainst the ingress of, for example, oxygen and moisture, where no thickcathode layer is present.

It will be understood by a person skilled in the art that the presentinvention is not limited to the described embodiment but can be modifiedin many different ways within the scope of the invention as defined inthe appended claims.

1. A method of fabricating an organic light-emitting device, comprisingthe steps of: forming a first conductive layer over a substrate; formingat least one layer of a light-emissive organic material over the firstconductive layer; forming a barrier layer comprising a dielectric overthe at least one organic layer; and forming a sputtered secondconductive layer over the barrier layer.
 2. A method of fabricating anorganic light-emitting device according to claim 1, wherein the secondconductive layer is patterned.
 3. A method of fabricating an organiclight-emitting device according to claim 2, wherein the step of formingthe patterned second conductive layer comprises deposition through ashadow mask.
 4. A method of fabricating an organic light-emitting deviceaccording to claim 2, wherein the step of forming the patterned secondconductive layer comprises the steps of: forming a layer of aphotoresist over the barrier layer; patterning the layer of photoresistto expose regions of the barrier layer where the second conductive layeris to be formed; depositing a conductive layer over the patterned layerof photoresist; and removing the regions of the conductive layer whichoverlie the patterned layer of photoresist.
 5. A method of fabricatingan organic light-emitting device according to claim 4, furthercomprising, prior to the step of depositing the conductive layer, aplasma cleaning step to remove any residual photoresist.
 6. A method offabricating an organic light-emitting device according to claim 2,wherein the step of forming the patterned second conductive layercomprises the steps of: forming a layer of conductive material; forminga layer of a photoresist over the layer of conductive material;patterning the layer of photoresist to expose regions of the conductivelayer; removing the exposed regions of the conductive layer; andremoving the photoresist.
 7. A method of fabricating an organiclight-emitting device according to claim 2, wherein the secondconductive layer is deposited by DC magnetron or RF sputtering.
 8. Amethod according to claim 1, wherein the sheet resistance of the barrierlayer is at least 1 MΩ/square.
 9. A method according to claim 1, whereinthe dielectric comprises an inorganic oxide.
 10. A method according toclaim 9, wherein the inorganic oxide is selected from the groupconsisting of an oxide of Al, Ba, Ca, Mg, Ni, Si, Ti or Zr, an oxide ofan Al alloy, or an oxide of Al—Li or Al—Mg.
 11. A method according toclaim 9, wherein the oxide is sub-stoichiometric.
 12. A method accordingto claim 1, wherein the dielectric comprises a carbide.
 13. A methodaccording to claim 12, wherein the carbide is selected from the groupconsisting of a carbide of Hf, Mo, Nb, Ta, Ti, W or Zr.
 14. A methodaccording to claim 1, wherein the dielectric comprises a boride.
 15. Amethod according to claim 14, wherein the boride is selected from thegroup consisting of a boride of Cr, Mo, Nb, Ti, W or Zr.
 16. A methodaccording to claim 1, wherein the dielectric comprises a nitride.
 17. Amethod according to claim 16, wherein the nitride is selected from thegroup consisting of a nitride of Ti or Zr.
 18. A method according toclaim 1, wherein the dielectric comprises a fluoride.
 19. A methodaccording to claim 18, wherein the fluoride is selected from the groupconsisting of a fluoride of Ca or Mg.
 20. In a method of producing anorganic light-emitting device including the steps of: forming a firstconductive layer over a substrate, forming at least one layer of organicmaterial over the first conductive layer, and sputter-depositing asecond conductive layer over the at least one layer of organic material,the improvement comprising: forming an intermediate barrier layercomprising a dielectric between the second conductive layer and the atleast one layer of organic material for protecting the at least onelayer of organic material during the step of sputter-depositing thesecond conductive layer.
 21. A method according to claim 20, wherein theintermediate layer is a fluoride layer.
 22. A method according to claim20, wherein the second conductive layer is patterned.
 23. A method offabricating an organic light-emitting device, comprising the steps of:forming a first conductive layer over a substrate; forming at least onelayer of a light-emissive organic material over the first conductivelayer; forming a barrier layer over the at least one organic layer,wherein the sheet resistance of the barrier layer is at least 1MΩ/square; and forming a sputtered second conductive layer over thebarrier layer.
 24. A method of fabricating an organic light-emittingdevice, comprising the steps of: forming a first conductive layer over asubstrate; forming at least one layer of a light-emissive organicmaterial over the first conductive layer; forming a barrier layercomprising a dielectric over the at least one organic layer, wherein thedielectric comprises an inorganic oxide; and forming a sputtered secondconductive layer over the barrier layer.
 25. A method according to claim24, wherein the inorganic oxide is selected from the group consisting ofan oxide of Al, Ba, Ca, Mg, Ni, Si, Ti or Zr, an oxide of an Al alloy,or an oxide of Al—Li or Al—Mg.
 26. A method according to claim 24,wherein the oxide is sub-stoichiometric.
 27. A method of fabricating anorganic light-emitting device, comprising the steps of: forming a firstconductive layer over a substrate; forming at least one layer of alight-emissive organic material over the first conductive layer; forminga barrier layer comprising a dielectric over the at least one organiclayer, wherein the dielectric comprises a carbide; and forming asputtered second conductive layer over the barrier layer.
 28. A methodaccording to claim 27, wherein the carbide is selected from the groupconsisting of a carbide of Hf, Mo, Nb, Ta, Ti, W or Zr.
 29. A method offabricating an organic light-emitting device, comprising the steps of:forming a first conductive layer over a substrate; forming at least onelayer of a light-emissive organic material over the first conductivelayer; forming a barrier layer comprising a dielectric over the at leastone organic layer, wherein the dielectric comprises a boride; andforming a sputtered second conductive layer over the barrier.
 30. Amethod according to claim 29, wherein the boride is selected from thegroup consisting of a boride of Cr, Mo, Nb, Ti, W or Zr.
 31. A method offabricating an organic light-emitting device, comprising the steps of:forming a first conductive layer over a substrate; forming at least onelayer of a light-emissive organic material over the first conductivelayer; forming a barrier layer comprising a dielectric over the at leastone organic layer, wherein the dielectric comprises a nitride; andforming a sputtered second conductive layer over the barrier layer. 32.A method according to claim 31, wherein the nitride is selected from thegroup consisting of a nitride of Ti or Zr.
 33. A method of fabricatingan organic light-emitting device, comprising the steps of: forming afirst conductive layer over a substrate; forming at least one layer of alight-emissive organic material over the first conductive layer; forminga barrier layer comprising a dielectric over the at least one organiclayer, wherein the dielectric comprises a fluoride; and forming asputtered second conductive layer over the barrier layer.
 34. A methodaccording to claim 33, wherein the fluoride is selected from the groupconsisting of a fluoride of Ca or Mg.
 35. In a method of producing anorganic light-emitting device including the steps of: forming a firstconductive layer over a substrate, forming at least one layer of organicmaterial over the first conductive layer, and sputter-depositing asecond conductive layer over the at least one layer of organic material,the improvement comprising: forming an intermediate barrier layerbetween the second conductive layer and the at least one layer oforganic material for protecting the at least one layer of organicmaterial during the step of sputter-depositing the second conductivelayer, wherein the intermediate layer is a fluoride layer.